Gas streams from industrial chemical processes may contain a variety of sulphur compounds such as H2S, SO2, CS2 and COS. The removal of these sulphur compounds is of considerable importance, because the toxicity and smell of sulphur compounds renders their presence highly undesirable. In addition, other compounds whose presence is undesirable may be present such as HCN or NH3. It is often necessary for the intended purposes of the gas stream to remove these compounds at least partly, for example to prevent catalyst poisoning in processes after the gasification stage.
Processes for the removal of sulphur compounds are known in the art.
In EP 324,526 a process is described for the removal of H2S, COS, CS2 and SO2 from a gas stream, by converting COS and CS2 into H2S. Although EP 324,526 mentions the removal of H2S, it does not specify the preferred type of process for H2S removal, nor is the concentration of H2S in the treated gas stream mentioned.
A process known in the art for the removal of H2S from a gas stream is the partial oxidation of H2S to SO2 according to:Partial H2S oxidation 2H2S+3O2→2H2O+2SO2   (1)The SO2 formed can be (catalytically) converted to elemental sulphur according to the Claus reaction:Claus reaction 4H2S+2SO2⇄4H2O+3S2   (2)This process is known as the Claus process. The Claus process is frequently employed both in refineries and for the processing of H2S recovered from natural gas.
A disadvantage of the Claus process is that it is thermodynamically limited by the increase of the water vapour content and simultaneously by the decrease of the H2S and SO2 concentration as the H2S conversion proceeds, with the result that the equilibrium of the Claus reaction shifts to the left. Since the dew point of water lies below the solidification point of sulphur, condensation of the water vapour in the process gas to remove this limitation can lead to serious problems, such as clogging due to the solidification of sulphur and corrosion due to the formation of sulfuric acid, causing serious operational problems.
At relatively low concentrations of H2S, typically concentrations below 20 vol %, the Claus process is difficult to operate and requires special technical provisions.
Another disadvantage of the Claus process is that the concentration of H2S in the treated gas is still relatively high. The Claus process has been improved to an extent where the H2S content of the treated gas can be. lowered to approximately 1 vol % (Superclaus-99 process) and the SO2 content to approximately 0.02 vol %. However, especially in the case where it is necessary to comply with requirements with regard to residential use or environmental regulations with respect to emission of sulphur compounds, even lower concentrations of H2S and SO2, in the ppm range, have to be achieved. A disadvantage of the Claus and Superclaus process is that such low concentrations of H2S and SO2cannot be achieved.
The removal of HCN from gas streams is important not only because of its own toxic properties, but also in view of corrosive NOX compounds which can evolve when both HCN and oxygen are present in a gas stream. In addition, HCN itself is corrosive to equipment when allowed to concentrate in liquid streams upon gas treating.